As a drive circuit of sensorless motor, for example a circuit driving a three-phase brushless motor is known.
This circuit for driving a three-phase brushless motor does not use a Hall element or other circuit detection element, but utilizes an induction voltage (inverse electromotive force or EMF (electromotive force)) generated in the drive coils of the three phases and switches the drive current supplied to the drive coils of the three phases. The drive circuit of this type of general sensorless motor detects the induction voltage of the excitation coils of the three phases, gives a certain delay amount to a timing when a polarity is inverted, and switches application of the drive current supplied to the drive coils of the three phases (supply of power).
Further, the drive circuit of a general sensorless motor eliminates a spike voltage (flyback voltage) generated at the time of the switching of the supply of power by a filter etc. Further, the drive circuit of a general sensorless motor prepares for a case where the rotor of the motor is already at a location where it should stop (also referred to as the “reference position”) and therefore the rotor of the motor is not started up immediately after the supply of power to the drive coils of the three phases by generating a startup pulse and forcibly switching to a power supply pattern of the drive coils of three phases when it is not possible to detect the induction voltage in the drive coils of the three phases within a certain time.
A delay amount is given to the power supply timing of such a drive current inverting in polarity. A filter for eliminating the spike voltage is provided. The signal processing systems for generating the startup pulse can be roughly classified into analog systems and digital systems.
A motor drive circuit of the analog system utilizes a CR time constant to eliminate the delay amount of the phase and the spike voltage and generate a startup pulse. A motor drive circuit of the digital system uses a microprocessor etc. to perform processing for functions equivalent to the analog system. Accordingly, a system having a large circuit scale can use the digital system, but applications where the circuit must be made small in scale cannot use a microprocessor etc. due to costs and the mounting area. Therefore, it has to use not the digital system, but the analog system.
The analog system has the defects that it is necessary to set the optimum constant of each element of the CR time constant circuit, but the setting the optimum constant is difficult since there are constant values which interfere with each other between elements. Further, large numbers of resistors and capacitors are necessary, therefore the number of parts becomes large.
In recent years, the vibration motors used as silent notifying means in electronic devices such as mobile phones for the purpose of use in noisy environments and of use by acoustically handicapped persons are being required to be made smaller in size, smaller in thickness, and higher in reliability. The motor generally used at the present is a brush-type coreless motor, but this motor has the problem of a short service life due to wear of the brush and the problem that the outside dimensions are hard to reduced in the diameter direction due to the structural requirement that air gaps must be provided inside and outside the rotating coil.
For this reason, elimination of the brush by fixing the coil of the vibration motor and using a permanent magnet as the rotor has been studied. A brushless motor does not have a brush, therefore has a high reliability and is advantageous for the reduction of thickness if making the motor structure a facing magnetic surface type. When viewing a brushless motor from the viewpoint of the design of the overall electronic device, there are requests for incorporating the drive circuit in the module of the motor. Note that naturally operational functions equivalent to the present brush-type coreless motors are required at this time.
In order to incorporate a drive circuit in a small-sized brushless motor, the formation of the drive circuit as an IC, the minimizing of the size of the IC leading to the reduction of the mounting area of the IC, a small number of externally attached electric parts other than the IC, and sufficiently small outer shapes of the externally attached electric parts are requested.
A drive circuit of a brushless motor using a phase locked loop (PLL) circuit is known (see for example Patent Document 1).
Patent Document 1 discloses a motor drive circuit and a motor drive method for the sensorless drive of a brushless DC motor by a simple circuit.
In Patent Document 1, a phase loop is formed so as to make the frequency output phase of the VCO determining the oscillation frequency match with the EMF phase. This becomes a so-called PLL circuit. The rotation phase of the rotor and the drive circuit phase are matched.
Patent Document 1: Japanese Patent Publication (A) No. 2001-061291